Molar mass distribution of linear and branched polyurethane studied by size exclusion chromatography

Linear polyoxypropylene (POP) diol and star POP triol were end linked using diisocyanate. Linear and branched polyurethanes (PU) of varying size were formed by varying the stoichiometric ratio r of isocyanate and hydroxyl groups. End-Linked POP triol forms a gel for r > 0.55. Molar mass distributions of linear and soluble branched PU were determined using size exclusion chromatography as a function of r in the range 0-1. The fractions of the first few oligomers at different r are in good agreement with calculated values using mean field theory. Branched POP triol has a very broad molar mass distribution close to the gel point. At low degree of polymerization (DP < 10) the molar mass distribution is well described by mean field theory while for DP > 300 it is better described by the percolation model. The results imply that many earlier measurements on gel-forming systems reported in the literature were done on samples in the crossover regime.